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521.
ELID超精密镜面磨削钝化膜状态变化的研究 总被引:1,自引:0,他引:1
建立了ELID磨削系统的电解反应回路等效模型,提出用极间电流表征金属结合剂砂轮表面的钝化膜状态,分析了钝化膜在ELID超精密镜面磨削电解预修锐阶段、在线电解修整动态磨削阶段和光磨阶段的状态变化。 相似文献
522.
J. Büchner 《Space Science Reviews》2006,122(1-4):149-160
Current sheets are essential for energy dissipation in the solar corona, in particular by enabling magnetic reconnection.
Unfortunately, sufficiently thin current sheets cannot be resolved observationally and the theory of their formation is an
unresolved issue as well. We consider two predictors of coronal current concentrations, both based on geometrical or even
topological properties of a force-free coronal magnetic field. First, there are separatrices related to magnetic nulls. Through
separatrices the magnetic connectivity changes discontinuously. Coronal magnetic nulls are, however, very rare. Second, inspired
by the concept of generalized magnetic reconnection without nulls, quasi-separatrix layers (QSL) were suggested. Through QSL
the magnetic connectivity changes continuously, though strongly. The strength of the connectivity change can be quantified
by measuring the squashing of the flux tubes which connect the magnetically conjugated photospheres.
We verify the QSL and separatrix concepts by comparing the sites of magnetic nulls and enhanced squashing with the location
of current concentrations in the corona. Due to the known difficulties of their direct observation, we simulated coronal current
sheets by numerically calculating the response of the corona to energy input from the photosphere, heating a simultaneously
observed Extreme Ultraviolet Bright Point. We did not find coronal current sheets at separatrices but at several QSL locations.
The reason is that, although the geometrical properties of force-free extrapolated magnetic fields can indeed hint at possible
current concentrations, a necessary condition for current sheet formation is the local energy input into the corona. 相似文献
523.
首先从飞机防火角度,用对称分量法分析了航空电机在不对称电源下的发热情况,推导了发热电流的公式,以此作为数学模型,解释缺相时电机很快烧毁的物理现象。最后提出了防止航空三相电动机过热与起火的方法,包括采用电机在线状态监控、主动维护、增加有效的保护装置等。 相似文献
524.
Effects of space weather on high-latitude ground systems 总被引:1,自引:0,他引:1
Risto Pirjola 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2005,36(12):2231-2240
Geomagnetically induced currents (GIC) in technological systems, such as power grids, pipelines, cables and railways, are a ground manifestation of space weather. The first GIC observations were already made in early telegraph equipment more than 150 years ago. In power networks, GIC may saturate transformers with possible harmful consequences extending even to a collapse of the whole system or to permanent damage of transformers. In pipelines, GIC and the associated pipe-to-soil voltages may enhance corrosion or disturb surveys associated with corrosion control. GIC are driven by the geoelectric field induced by a geomagnetic variation at the Earth’s surface. The electric and magnetic fields are primarily produced by ionospheric currents and secondarily affected by the ground conductivity. Of great importance is the auroral electrojet with other rapidly varying currents indicating that GIC are a particular high-latitude problem. In this paper, we summarize the GIC research done in Finland during about 25 years, and discuss the calculation of GIC in a given network. Special attention is paid to modelling a power system. It is shown that, when considering GIC at a site, it is usually sufficient to take account for a smaller grid in the vicinity of the particular site. Modelling GIC also provides a basis for developing forecasting and warning methods of GIC. 相似文献
525.
526.
在航空发动机控制系统中,存在着大量直流驱动负载,如力矩马达、电液伺服阀等。高精度,高稳定性的直流驱动电路对发动机控制系统的性能有着至关重要的影响。针对以往采用分立元器件设计的电路,精度较低,电路设计复杂,采用集成元器件设计直流驱动电路,提高了系统精度,简化了电路设计,同时降低了设备的重量和体积。 相似文献
527.
528.
介绍了大电流充放电电源电流校准装置的组成和工作原理,根据JJF1059-1999《测量不确定度评定与表示》关于不确定度评定的原则,对电流校准的不确定度进行了分析和评定。 相似文献
529.
530.
S.Yu. Bobrovnikov I.I. Alexeev E.S. Belenkaya V.V. Kalegaev C.-R. Clauer Ya.I. Feldstein 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2005,36(12):2428-2433
We compute global magnetospheric parameters based upon solar wind data obtained from the WIND spacecraft upstream. Using the paraboloid magnetospheric model, calculations of the dynamic global magnetospheric current systems have been made. The solar wind dynamic pressure, the interplanetary magnetic field, the strength of the tail current, and the ring current control the polar cap and auroral oval size and location during the magnetic storm. The model calculations demonstrate that the polar cap and the auroral oval areas are mainly controlled by the tail current. The substorm onset at 0630 UT on September 25, 1998 happened near the minimum in the main phase field depression. The substorm expansion onset time is also marked by a sudden enhancement in the solar wind dynamic pressure and an enhancement in the tail current. The magnetic signatures of these two effects cancel each other, which explains why the Dst profile shows no strong time variation during the substorm. Evidence for the substorm expansion includes not only the signature in the AL index but also the strong asymmetry of the low latitude magnetic disturbances (substorm positive bay signature). Model calculations were checked by comparison with the GOES 8 and 10 magnetic field measurements. 相似文献